Thermoplastic polymers are widely employed to create a variety of products, including blown and cascade films, extruded sheets, foams, fibers and products made therefrom, woven and knitted fabrics, and non-woven fibrous webs. Many thermoplastic polymers used in these products, such as polypropylene, are inherently hydrophobic, and there are a number of uses for thermoplastic polymers where their hydrophobic nature either limits their use or requires some effort to modify the surface of the shaped articles made therefrom. For example, polyolefins are used in the manufacture of nonwoven webs that are employed in the construction of absorbent articles such as diapers, feminine care products, and personal incontinence products, and the use of such articles self-evidently are limited because of their hydrophobic nature.
When fiber, and the fabrics made therefrom, is still hydrophilic after drying following contact with deionized water, the fiber or fabric is considered durably hydrophilic. Hydrophilic fiber may be obtained by topically spraying or coating the hydrophobic fiber with certain surfactants and subsequently drying the fiber or fabric. Typically, however, the surfactant that remains on the fiber using this technique is diminished or lost completely upon contact with an aqueous medium, e.g. water, and thus the hydrophilicity of the fibers is only poorly durable. Hydrophilicity, or the lack thereof, can be measured in a variety of ways. For example, when water contacts a nonwoven web that has lost its hydrophilicity, the water does not flow, or flows undesirably slowly, through the web.
WO 92/18569 and WO 95/01396 describe fluorochemical additives for use in the extrusion of thermoplastic polymers to prepare films and fibers with repellency properties. It is specifically disclosed that films can be prepared with good anti-wetting properties. It is further taught that polypropylene films with the fluorochemical dispersed therein have good antistatic properties.
The addition of one or more surfactants to the melts of thermoplastic polymers to impart hydrophilicity to both the surface and the bulk of the fiber is also taught in the art. U.S. Pat. Nos. 4,857,251 and 4,920,168 (Nohr et al.) describe a method of forming fibers by melt-extrusion of a surface segregatable thermoplastic composition that comprises thermoplastic polymer and siloxane-containing additive having certain moieties. After the fibers are formed, they are heated from 27° C. to 95° C. for a period of time sufficient to increase the amount of additive at the fiber surface. The resulting fibers exhibit increased surface hydrophilicity compared to fibers prepared from the thermoplastic alone.
U.S. Pat. No. 5,244,951 (Gardiner) describes a durably hydrophilic fiber comprising thermoplastic polymer and fluoroaliphatic group-containing non-ionic compound dispersed within said fiber and present at the surface of the fiber.
U.S. Pat. No. 5,804,625 (Temperante et al.) discloses the addition to the polymer melt of a blend of one or more nonionic fluorochemical surfactants and one or more nonionic, non-fluorinated, poly(oxyethylene) group containing surfactants to impart durable hydrophilicity to the surface of an article extruded from the polymer. Particular articles that can be produced and provided with durable hydrophilicity at the surface include fibers, fabrics and films.
EP 0 516271 discloses the use of a fluoroaliphatic group containing nonionic compound in the extrusion of polypropylene fibers to impart durable wettability properties to the fiber's surface. Particularly exemplified compounds are those that have a perfluorinated alkyl group linked to a poly(oxyalkylene) group terminated with a hydroxyl group or a lower alkyl ether.
Coating methods to provide a hydrophilic surface are known, but also have some limitations. First of all, the extra step required in coating preparation is expensive, time consuming and involves safety and environmental issues. Many of the solvents used for coating are flammable liquids or have exposure limits that require special production facilities. Furthermore the quantity of surfactant is limited by the solubility in the coating solvent and the thickness of the coating.